Photodissociation dynamics of nitrobenzene and o-nitrotoluene.

Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO(2) elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO(2) elimination channels were determined to be NONO(2)=0.32+/-0.12 (193 nm), 0.26+/-0.12 (248 nm), and 0.4+/-0.12(266 nm) for nitrobenzene and 0.42+/-0.12(193 nm) and 0.3+/-0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results.